A transmit diversity communication device is one having a plurality of transmit paths that simultaneously transmit respective transmit signals that differ by a transmit diversity signal, for example, a relative phase, relative amplitude, or relative power. One goal of mobile transmit diversity is to obtain constructive interference, also known as beamforming, of the transmit signals at the receiver, e.g., a base station.
In transmit diversity systems, one possible advantageous feature is an extension of operating range. This effect may be an outcome of forming a beam using the plurality of transmit paths that exhibits a higher Effective Radiated Power (ERP) at the base station than a conventional (non-diversity) transmitter transmitting using a single transmit path. Transmit beamforming can be performed by using feedback information from the receiver to the transmit diversity transmitter in adjusting the value of the transmit diversity parameter.
Generally, in communication systems between a mobile handset, for example, a mobile telephone, personal digital assistant (PDA), laptop computer, etc. and a base station, there is typically a protocol for mobile communication with the base station.
As described in prior patent applications of the assignee of the present invention, a mobile transmit diversity (MTD) device may use two antennas simultaneously transmitting with a transmit diversity parameter, for example, a phase difference or a power ratio between at least two antennas. In one embodiment, two power amplifiers may be used to amplify the respective transmit signals, thereby potentially providing a current (i.e., power) saving by the mobile device, as described, for example, in U.S. Pat. Pub. No. 2007/0218849, the entire contents of which are incorporated herein by reference. Use of mobile transmit diversity may permit extending the effective range of a mobile device from a base station using the same (or less) power as a non-diversity transmission device.
As described therein, the transmitter chains may include a power amplifier (PA), a phase modulator (PM) and an antenna. When configured to provide beam forming MTD, the relative phase between the two paths may be adjusted such that the signals arrive at the base station antenna in-phase to constructively add. In this case the performance is higher than achieved with just the sum of the two powers combined. The difference is diversity gain (Gd).
Because a mobile transmit diversity device may obtain a diversity gain at the receiver using feedback to provide beamforming, current consumption efficiency gains may be produced. For example, a suitable receive power may be obtained by operating one or more of the amplifiers at less than half the specification power of the device. Accordingly, one or both of the amplifiers may be operated to provide maximum efficiency at a fraction, e.g., half or even a quarter, of the specification power of the device.